The neuroblastoma is a tumor which exhibits many features characteristic of mature neurons. Cells isolated from the tumor have been adapted to growth in tissue culture, and therefore provide an excellent model system for studying cellular differentiation in vitro. The purpose of this investigation is to explore the means of modifying both tumorigenicity and the capacity for differentiation in cultured mouse neuroblastoma cells. Specifically, clones of neuroblastoma cells with marked differences in the degree of morphological and biochemical differentiation will be examined both by conventional radioactive labeling techniques and by electron microscopy for the presence of intracellular virus particles. In addition, growing cells will be treated with a number of chemical agents: halogenated pyrimidines (BrdU, FdU, and IdU), dimethyl sulfoxide, and dibutyryl cyclic adenosine monophosphate. These experiments will be carried out in an attempt to induce significant increases in virus production and to relate these increases to cell differentiation. Experiments will be carried out to mutagenize cells and derive clones producing high levels of viral particles. Lastly, we hope to determine whether the expression of viral synthesis can be correlated with the tumorigenic potential of the neuroblastoma cell.